Shadow glitches in Cycles
Have you tried recalculating normals? I often get weird artifacts/shadows when the normals get flipped around.

Tab into edit mode with your mesh selected
Select all the faces of the mesh with A
For blender 2.8 SHIFT + N brings up the recalculate normals tool. You want to &quot;Recalculate Outside&quot;.

Or you can access this via:
Edit Mode &gt; A &gt; Mesh menu &gt; Normals &gt; Recalculate Outside

https://all3dp.com/2/blender-recalculate-normals-simply-explained/#:~:text=There%20are%20two%20main%20ways,so%20they%20all%20face%20outwards.
Explanation of normals from All3DP:

Ever wondered why your Blender model is sometimes plagued by random
black voids?
The normal of a face describes the direction it’s pointing or facing.
Usually, in a closed model, all normals should be facing outwards.
However, sometimes when manually manipulating edges and faces in
Blender, the normals get flipped. It’s not the end of the world, but
it does make your model look weird. And once you export the model,
flipped normals can leave behind bad artifacts.
That’s why it’s best to fix these flipped normals. And the good news
is, they’re super easy to recalculate.